Printed circuit board assemblies are ubiquitously employed in a wide range of products ranging from mass scale consumer products to sophisticated instruments used for specialized applications. It is highly desirable that printed circuit board assemblies, particularly ones that are used in mass scale consumer products, are manufactured in a cost-effective manner and are highly reliable to operate. Towards this end, most manufacturers of printed circuit board assemblies use automated machinery that execute various functions such as picking and placing components upon a printed circuit board, followed by soldering these components upon the printed circuit board. The completed printed circuit board assembly is then inspected for quality by various types of inspection machines that carry out the inspection in a very rapid and efficient manner. One of the inspection machines is known in the industry as a bed-of-nails tester and is used to test the integrity of various soldered connections as well as to detect certain types of failures in the components mounted upon the printed circuit board.
The types of testing performed by the bed-of-nails tester may include various functional tests to evaluate various circuit functions of the printed circuit board assembly. In one exemplary functional test, a voltage gain of an amplifier circuit may be evaluated by applying power to the amplifier circuit and measuring an output signal that is generated by the amplifier circuit in response to a test signal coupled into the amplifier circuit. In some cases, such a functional test may fail to evaluate circuits other than the amplifier circuit on the printed circuit board assembly and may also fail to verify connectivity between various nodes on the printed circuit board assembly that are not associated with the amplifier circuit.
Consequently, functional tests are often complemented, or supplemented, by continuity tests that are used to check for abnormal connections such as short circuits and/or open circuits. A short circuit condition can occur for example, when an excessive amount of solder causes a solder bridge to be formed between two solder pads on the printed circuit board assembly. Abnormal open circuit conditions can occur for example, when no solder is applied to a solder pad, or when an inadequate amount of heat is applied for melting the solder on to the solder pad.
The continuity tests can be carried out in various ways such as by propagating a DC current from a solder pad located on a bottom surface of a printed circuit board assembly and through a lead of a device that is mounted upon a top surface of the printed circuit board assembly to verify the solder connection. The continuity test works effectively for evaluating such a connection. However, there may be several elements on the top surface of the printed circuit board assembled that may be inaccessible for purposes of propagating a DC current. One example of such an inaccessible element is a solder joint located below a ball-grid array package mounted upon a printed circuit board.
The connections associated with the ball-grid array package are therefore tested by placing a test electrode on a top surface of the ball-grid array package for receiving an AC signal that is propagated from a solder pad on the printed circuit board assembly and through the body of the ball-grid array package. Such a test configuration may prove satisfactory when the capacitive coupling provided through the body of the ball-grid array package is adequate for propagating the AC signal. However, this is not always the case because of various factors such as for example, a large and unavoidable separation distance between the test electrode and a pin of the ball-grid array package through which the AC signal is propagated into the ball-grid array package by the bed-of-nails tester.